I have deco m5 with three nodes. 
 

My current setup is as follows (wired backhaul) 

Spectrum modem (arris sb6183) -> main deco--> network unmanaged switch (tl-sg108s) ---> two additional deco nodes (each coming from the switch). 
 

I did some research and what I understand is access point is what I should be using as the operation mode

Through the deco app (advanced) I changed  operation mode to access mode and all the decos go solid red. I restarted modem and switch and all the decos... same result. 
 

I read somewhere that I might need a root router between modem and main deco? Not sure. If I do, can I just use another deco node as that router?

modem-> deco as router-> main deco-> switch-> two additional decos in parallel. 
 

I'm a novice at networking so please be patient with me. Thanks